# At what speed does information move through the atoms of a rigid object?

How fast does information travel on particles? I thought if you move a iron bar from one end it would take the speed of sound to move its other end. For example, theoretically if you hold an iron bar that has the length from the earth to the moon it would take 18 hours to move the other end. However on this video at 3:00 it’s said that the mechanical clock arm atoms transfer the movement information at the speed of light? What's the difference?

A mechanical impulse will travel along an iron bar at the speed of sound (which will depend on the material and also possibly on the frequency and amplitude of the impulse). But in the video the narrator is talking about a photon travelling along the length of the iron bar, and he is assuming that the photon travels at the speed of light in vacuum - which, of course, is unrealistic if the photon is inside the iron bar. Basically, you are finding inconsistencies in a not-very-good YouTube video.

• Considering that this is the highest-voted answer at the moment, it might be sensible to add that the speed of sound depends on the material - probably very obvious to most people, but still. Also, I find it kind of a roundabout way to talk about this in terms of "sound"; the deciding factor is the "speed of pressure waves" in the material at hand, which of course equates to sound if/when it's a gas or liquid and ears are involved. ;)
– AnoE
Commented Nov 27, 2023 at 11:45
• @AnoE Thanks for the suggestion - I have edited my answer. Commented Nov 27, 2023 at 11:47
• @AnoE I can still recall being surprised at the speed of sound in a material showing up in calculations of heat transport by phonons. It's certainly counterintuitive Commented Nov 27, 2023 at 12:24
• @JFrank I would bet that the error comes from the fact that the internal forces in solids are electromagnetic in nature: It is indeed electrons that communicate the change in relative movement between the atoms of the iron bar. But that doesn't mean that the acceleration imposed is transferred at this speed. Commented Nov 27, 2023 at 18:11
• Is "moving" a rod really the same as "banging" it? Same difference as applying pressure to a hydraulic fluid vs. having it conduct (sound) waves. It's not immediately clear to me that the two are the same. Commented Nov 28, 2023 at 13:58

The question in the title is formally contradictory. A model is always a simplification of the real world for the sake of understanding and making predicions (usually mathematically) A "rigid object" is a mathematical model of an object that simplifies away things like atoms, so "atoms of a rigid object" is an oxymoron.

Rigid bodies are a useful model for Newtonian mechanics, but they don't work for Special Relativity. In relativity, a signal can't travel faster than the speed of light, so it might be useful to model a "maximally rigid" body, in which the motion is transmitted at this speed. The transmission of movement from the centre of a clock to its tips is limited by the speed of light.

For actual practical clocks, the really transfer of motion happens much much slower, at the speed of sound in the substance. But this is a mass matter property. It is something that only appears when there are billions of atoms and their movements can be coordinated, not a fundamental fact about the universe.

Though I do wonder if thinking about time dilation (the least intuitive part of SR) is a good way gain intuition about the speed of light (something that seems more plausible)

• So if the transmission of movement on the arms of a mechanical clock still happens at speed of sound, why the guy in the video says it’s at speed of light? Commented Nov 26, 2023 at 13:49
• He is using a model of a message being passed along a single line of atoms. He isn't really talking about a physical bar of iron. It's a rather odd video... To get intuition on the speed of light from time dilation seems the wrong way round. Time dilation would seem to be the highly counter intuitive concept. Commented Nov 26, 2023 at 15:54